Downy mildew is caused by parasitic fungi of the family Peronosporaceae, a group of water molds. The resulting disease is quite common in humid climates, affecting most of the fruits, vegetables, flowers, and grasses that gardeners like to grow.
This mildew tends to overwinter in old plant matter. Unfortunately, it also has a remarkable ability to survive in the soil for long periods of time, sometimes years. It can spread via wind, rain, insects, or infected seeds and cuttings.
- Stunted growth.
- White to purple down on the underside of leaves and stems.
- Pale, sometimes speckled, upper surfaces of leaves.
- Distorted leaves.
- Leaf drop.
- Discolored radishes.
- Distorted roots and flowers on beets.
- Leathery, wrinkled grapes.
- Mottled lima bean pods.
- Rapid plant death.
Treating downy mildew is difficult and not worthwhile in most cases. Destroy the affected plants immediately, taking care to avoid bringing them into contact with healthy plants. Apply lime or sulfur to the area where the affected plants were growing to kill any remaining fungus.
For perennial fruits, such as grapes, fungicides can be used to save the plants. Bear in mind, however, that this method of control is not very effective. Proactively spraying the plants before infection sets in will be necessary in the future. Organic growers can use a copper spray as a natural fungicide.
Practicing good garden sanitation is key to preventing this disease:
- Raise your own seedlings instead of buying transplants.
- Plant seeds or seedlings in the garden only when the soil temperature is suitable for that particular type of plant.
- Plant seeds at the spacing recommended on the seed packet.
- Practice crop rotation.
- Take steps to improve soil drainage, if necessary.
- Do not overwater the garden.
If downy mildew is a recurring problem in your garden despite your efforts, buy only mildew-resistant plant varieties.
“Soil Temperature Conditions for Vegetable Seed Germination”
Useful factsheet listing minimum, maximum, and optimum soil temperatures for starting common vegetables.